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High temperature ethanol fermentation of lignocellulosic wastes

Exploitable results

A laboratory scale pilot plant is being developed to test a novel process, using a novel thermophile, for continuous ethanol production from a range of cheap and abundant agricultural wastes. The stage reached is that a suitable mutant thermophile has been selected and tested on pure xylose feedstocks in an open system that uses an anaerobic catalytic stage with cell recycle via hollow fibre ultrafiltration in which there is no cell growth. Long term continuous operation is achieved by feeding fresh cells grown on xylose aerobically to compensate for the death rate of cells in the catalytic reactor. The results show sucrose volumetric productivity although ethanol yields on xylose are slightly below those of the best yeasts on glucose. Ethanol costs are predicted to be well below those for petrochemical ethanol. The lack of a nonreverting strain has been the biggest handicap to the research. However despite this, significant progress has been made. The existing LLD-15 and LLD-16 strains were used in the open system to provide useful results on relative fermentability of various components in the real feedstocks. These have shown that almost all the sugar in various mixtures is efficiently fermented and no sign of toxicity from other components has been detected. The cell recycle system can be used over long periods even with these reverting strains to obtain product yields at moderate feedstock concentration and new neutral pH. It is likely that an LLD-deletion strain will shortly be available via the recombinant deoxyribonucleic acid (DNA) strategy enabling the feedstocks to produce reasonable ethanol yields at very high volumetric ethanol productivities, particularly in the closed system. The encouraging data from dilute acid hydrolysates of corn cobs and from enzymic hydrolysates of beet pulp suggest that the former strategy will be applicable to a range of high lignin wastes such as straw or wood chips, and the latter to low lignin wastes such as beet pulp, citrus wastes or enzymic retting of flax or rape straws.